Chatting to a photographer friend who has done a lot of portrait work, he later gave the best piece of advice you could ever give. Patience! Wait for the moment... the more you try to get the little dudes to look at the camera, the more they are going to want to run around pretending to be racing cars or elephants. And when one is smiling and happy, the other will do everything he can to add some comedy value to a near perfect shot. (Nothing I don't try to do when friends take pictures of me though).
Another try out with new situations again! The same as with my music and the Saxophone, I think its important to push your art and try to put yourself in situations you aren't really comfortable in. With photography I´m happiest when shooting live gigs, or portraiture. This isn't to say I don't enjoy all the other scenarios, just that thats´s where I feel most comfortable. So when I got the opportunity to capture toddler twins I jumped at the chance.... I now have a new found, super respect for full time child portrait photographers. How do you get them to keep still. Hahahah